1. Field of the Invention
This invention relates to conversion between analogue and digital signals. In particular, this invention relates to analogue to digital converters, digital to analogue converters, and digital audio processing apparatus incorporating such converters.
2. Description of the Prior Art
Analogue to digital (A/D) and digital to analogue (D/A) converters are used to convert between analogue and digital representations of a signal such as a digital audio signal.
One application of A/D and D/A converters is in a digital audio processing apparatus such as a digital audio mixing console. In this type of apparatus, analogue signals from audio sources (e.g. microphones or guitars) are converted by respective A/D converters into sampled digital audio signals. The digital audio signals (possibly together with further directly generated digital audio signals, for example from a digital synthesizer) are then mixed or combined in the mixing console using entirely digital techniques such as digital filtering. The resulting digital audio signals are then either converted back to the analogue domain by a D/A converter (to be heard by an audience, the artist or a mixing engineer) or recorded (for later D/A conversion).
A recently adopted technique in A/D and D/A conversion is to perform the conversion at a low resolution (e.g. 1 to 4 bits) but at a sampling rate (e.g. 64 Fs) which is many times greater than the actual sapling rate (Fs) of the required digital signal. The technique is described in the book "Digital Signal Processing" (Proakis and Manolakis, Macmillan Publishing, 1992). In an A/D converter using this technique, a digital filter is then used to decimate the 64 Fs signal to generate the required digital signal at the lower sampling rate (Fs) but a much higher resolution (e.g. 16 bits). A corresponding D/A converter uses an interpolation filter to oversample a high resolution Fs signal up to a low resolution 64 Fs signal before the oversampled low resolution signal is converted to an analogue signal.
It has been recognised that particularly faithful conversion between digital and analogue audio signals can be achieved using A/D and D/A converters of the type described above incorporating so-called linear phase response digital filters. These filters are described in the book referred to above. However, a disadvantage of linear phase response filters in some circumstances is the relatively long delay which they impose on the conversion process. This delay, which is of the order of about 2 mS (milliseconds) per conversion stage, can mean that a foldback audio signal played from the digital mixing console to an artist during a recording session or performance can be heavily delayed with respect to the artist's own voice or instrument. This delay (e.g. 4 mS) can be subjectively disturbing for the artist.